1. Field of the Invention
The present invention relates, most generally, to semiconductor structures, and more particularly to phase change memory cells.
2. Description of the Related Art
With advances in electronic products, semiconductor technology has been applied widely in manufacturing memories, central processing units (CPUs), liquid crystal displays (LCDs), light emitting diodes (LEDs), laser diodes and other devices or chip sets. In order to achieve high-integration and high-speed requirements, dimensions of semiconductor integrated circuits have been reduced and various materials, such as copper and ultra low-k dielectrics, have been proposed and are being used along with techniques for overcoming manufacturing obstacles associated with these materials and requirements.
Generally, memory includes volatile memory and nonvolatile memory. Volatile memory such as dynamic random access memory (DRAM) is provided to store data or information of a system. A DRAM cell may include a transistor and a capacitor. Due to its simple structure, costs for manufacturing DRAM are low and the process for manufacturing DRAM is easier than for non-volatile memories. However, when voltages applied to DRAM are turned off, data stored in DRAM will disappear. Also, DRAM cells must be periodically refreshed to maintain the data stored therein, because of current leakages from DRAM capacitors.
Nonvolatile memory such as Flash memory has been widely used to maintain data even if input voltages applied to the Flash memory are removed. Data stored in Flash memory can be desirably removed by UV radiation or electrical erasing. However, a Flash memory cell usually includes multiple gate structures for storing data and is more complex than a DRAM cell. Further, the erase/re-write cycle of Flash memory is another issue due to leakage currents from floating gates of the Flash memory cell. Accordingly, processes for manufacturing Flash memory are more difficult than for DRAM, and costs for manufacturing high quality Flash memory with more erase/re-write cycles are comparatively high.
Recently, various nonvolatile memories such as phase-change memory (PCM), magnetic random access memory (MRAM) or ferroelectric random access memory (FRAM) having cell structures that are similar to those of DRAM have been proposed.
FIG. 1 is a schematic equivalent circuit of a PCM cell.
Referring to FIG. 1, a PCM cell 100 includes a transistor 110 and a PCM capacitor 120. A source/drain (S/D) of the transistor 110 is coupled to ground, and another S/D of the transistor 110 is coupled to one end of the PCM capacitor 120. The gate of the transistor 110 is coupled to a gate voltage VG. Another end of the PCM capacitor 120 is coupled to a bit line voltage VBL.
When the data stored within the PCM capacitor 120 is to be accessed, the voltage VG is applied to and turns on the transistor 110, and the bit line voltage VBL is applied to the PCM capacitor 120, such that a read current may flow through the PCM capacitor 120 and the transistor 110. Based on the level of the output current, the data stored within the PCM capacitor 120 is accessed.
By changing the phase of a phase change material (not shown) within the PCM capacitor 120, the impedance of the PCM capacitor 120 may dramatically change. For example, the PCM capacitor 120 may have a low impedance and a read current (not shown) flowing through the capacitor 120 and the transistor 110 may be high. The low-impedance PCM capacitor 120 may store a data value of “1.” However, if the PCM capacitor 120 has a high impedance, a read current (not shown) flowing through the capacitor 120 and the transistor 110 is low, and the high-impedance PCM capacitor 120 may store a data value of “0.”
Since the PCM cell 100 includes one transistor 110 and one PCM capacitor 120, the PCM cell 100 is simple and the process for manufacturing the PCM cell 100 may be easier than that of Flash memory. In addition, PCM transistor 100 uses phase changes of the phase change material (not shown) to define the stored data “0” and “1.” The concern for current leakage of the PCM capacitor 120 may be substantially reduced.
Based on the foregoing, PCM structures are desired.